A cam shaft of an engine or a rotating shaft of an auxiliary engine part such as a pump shaft has a pulley mounted thereon to transmit a drive force to the cam shaft (or pump shaft) from a crankshaft of the engine by means of a belt engaged over the pulley. As illustrated in FIG. 4a of the accompanying drawings, a pulley "p1" includes a cylindrical rim "a" over which a belt is engaged, and a web "c" mounted on a rotating shaft "b" extending through a radially inside position of the rim "a". The pulley is generally fabricated by sintering or the like.
At ends of the rim "a" in an axial direction, provided are flanges "d" and "e" for preventing the belt on the rim "a" from falling off. Among the flanges "d" and "e", the left flange "d" is molded integrated with the rim "a" and the right flange "e" is provided as a separate element (flange plate) "f". This is because both of the flanges "d" and "e" cannot be molded together with the rim "a" as molding dies cannot be separated.
The flange "d" integrated with the rim "a" preferably faces the engine or auxiliary part (indicated by the chain line) when the pulley "p1" is mounted on the cam shaft of the engine or the rotating shaft "b" of the engine auxiliary part. If the flange "e" were integrated with the rim, the belt must go over the flange "e" in attaching the belt to the pulley or removing the belt from the pulley. This would deteriorate easiness in engine assembling and maintenance.
Incidentally, demand for engine size reduction and weight reduction has been increasing year by year. In order to meet the demand, the inventors developed a pulley "p2" as illustrated in FIG. 4b, where the pulley is made of resin and has a web "c" offset from the center of a rim "a" in the axial direction. The weight of this pulley "p2" is reduced by the use of resin, and a projecting length L of the pulley can be reduced by offsetting the web "c" when mounted on the engine (chain line).
However, the inventors found the following problems when they fabricated and studied a prototype (a part made on an experimental basis) of the pulley "p2".
(1) As illustrated in FIGS. 6a and 6b, a load was applied to a belt engaging surface of the rim "a" of each pulley "p1" and "p2" shown in FIGS. 4a and 4b, and deflection at the rim end A was measured. Measured data are shown in the diagram of FIG. 3. As understood from this diagram, the deflection of the pulley "p2" is considerably greater than that of the pulley "p1" when the same load is applied. This is because a moment exerted on the rim end A increases as a result of offsetting the web "c", and the rim "a" cannot bear the moment as a result of using resin for the pulley.
Therefore, there is a following possibility; if the pulley "p2" is mounted on the engine and the engine is operated, a belt tension acting on the rim "a" may cause the rim "a" to tilt and consequently the belt may come into uneven contact with the pulley "p2". This may damage the belt (breakage in the worst case) or result in uneven wear of the pulley "p2" (breakage in the worst case).
(2) As illustrated in FIG. 5, the pulley "p2" is molded by injecting resin into molding dies X and Y, which are divisible in the pulley axial directions, from a resin inlet "g" at the pulley rotation center.
When the inventors tried to mold the flange "d", which prevents the belt from falling off, together with the pulley "p2" in FIG. 4b, resin injected from the inlet "g" was cooled on its way to the flange "d" via the web "c" and rim "a" (see the upper half in FIG. 5), which resulted in a lot of defective units without the flange "d". This is because a distance from the resin inlet "g" to the flange "d" is longer than the pulley "p1" in FIG. 4a as a result of offsetting the web "c".
Consequently, it is practically required to later attach the flange "f" (FIG. 4a) even to the side of the pulley "p2" which faces the engine. Thus, the number of necessary parts increases and the manufacturing cost inevitably increases particularly when the pulleys "p2" are mass produced. If the flange is to be formed on the opposite side of the pulley "p2", it can integrally be molded, but easiness in assembling and maintenance of the engine is deteriorated as mentioned earlier.
An object of the present invention, which was developed in consideration of the foregoing, is to provide a pulley that is able to minimize deformation caused by belt tension acting on a rim of the pulley even if the pulley is made of less rigid material, i.e., resin.